Quantum Dot Chip on Board

ABSTRACT

Quantum dots used to modify the spectral output of an LED exhibit less of a performance decrease (due to increased temperature) when incorporated in a chip on board (COB) as compared to conventional LED packages. A ceramic ring may be used to shield the quantum dots from the heat associated with connecting electrical leads to pads on the COB. The upper surface of the ceramic ring may be sealed with a glass disk or other transparent material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/930,839 filed on Jan. 23, 2014.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to light emitting diodes (LEDs).More particularly, it relates to LEDs incorporating fluorescentmaterials in the form of quantum dot (QD) nanoparticles.

2. Description of the Related Art including information disclosed under37 CFR 1.97 and 1.98

Light emitting diodes (LEDs) have become a popular source for lightingbecause they use less energy than traditional incandescent lightsources. Dual in-line package light emitting diodes (DIP-LED) andsurface mounted diodes (SMD-LED) are two, popular, modern forms of LEDpackaging. However, these forms suffer from insufficient power densityfor some applications. Additionally, it is desirable to have LED devicesin smaller sizes than provided by those configurations.

Chip on board LED (COB-LED) offers an alternative to DIP-LED and SMDLEDpackaging. COB lighting provides the highest power density currentlyavailable in an LED device. Furthermore, the device may be manufacturedin a small and flat form factor. COB-LEDs are manufactured by attachingLED semiconductor chips directly to printed circuit boards. Directlyconnecting LEDs provides thermal management, high packaging density, andlong life with high performance.

LEDs do not emit white light. Therefore, if an LED is to be used as awhite light source, the light emission of the LED must be conditionedusing some additional phosphor. The first example of a white LED used ablue LED, a yellow LED, and a Y₃Al₅O₁₂:Ce (YAG) phosphor coating. Theemission frequencies of the LEDs, combined with the secondary emissionfrom the phosphor, yields white light as perceived by the human eye.Unfortunately, the white light produced by this combination is perceivedas cold and artificial and therefore, unpleasant to some people.Subsequently, many different phosphors have been used in an attempt togenerate white light more closely resembling the light produced bytraditional incandescent bulbs. On potential avenue is coating theseLEDs with nanocrystal semiconductors, also known as quantum dots, that,in combination, emit white light instead of traditional phosphors.

Quantum dots are semiconductor nanoparticles having dimensions on theorder of 2-10 nm. There is substantial interest in incorporating quantumdots into products because of their size-dependent, fluorescentproperties. To date, the majority of quantum dot formulations have beenmade of II-VI materials, namely, ZnS, ZnSe, CdS, CdTe; and most commonlyCdSe due to its tunability over the entire visible spectrum. Asmentioned earlier, quantum dots are of academic and commercial interestdue to their properties which differ from properties of correspondingcrystalline bulk forms of the same semiconductor material.

Two fundamental factors are responsible for the unique properties ofquantum dots. First, the surface-area-to-volume ratio of quantum dots isrelatively large. As a particle becomes smaller, the ratio of the numberof surface atoms to those in the interior increases. This is importantbecause the surface properties of the particle play a larger role in theoverall properties of the nanoparticle than in macro particles. Thesecond factor is that, with semiconductor nanoparticles, there is achange in the electronic properties of the material with the size of theparticle. Specifically, the band gap gradually becomes wider as the sizeof the particle decreases. This change in band gap is because of quantumconfinement effects. This effect is a consequence of the confinement ofan ‘electron in a box’; giving rise to discrete energy levels similar tothose observed in atoms and molecules rather than correspondingmacrocrystalline material. This leads to the important luminescentproperty of quantum dots: narrow bandwidth emission that is dependentupon particle size and composition.

The emission frequency of a quantum dot is inversely related to thedot's diameter. Therefore, as a quantum dot increases in size, thefrequency of emission decreases and, in the inverse, as quantum dotsdecrease in size emission frequency increases. Because quantum dots canbe manufactured with different diameters, quantum dots can be “tuned” toemit electromagnetic radiation in the various colors of the visiblespectrum.

BRIEF SUMMARY OF THE INVENTION

Quantum dots used to modify the spectral output of an LED exhibit lessof a performance decrease (due to increased temperature) when in a chipon board (COB) configuration as compared to conventional LED packages.

In certain embodiments of the invention, a ceramic ring is used toshield the quantum dots from the heat associated with connectingelectrical leads to pads on the COB.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is an exploded view of a quantum dot chip on board according tothe invention.

FIG. 2 is a cross-sectional view of the assembled device illustrated inFIG. 1.

FIGS. 3A and 3B are thermal scans of an LED in a conventional package.

FIGS. 4A and 4B are thermal scans of a quantum dot chip on boardaccording to the invention.

FIG. 5 is a graph showing the performance drop of quantum dots withincreasing temperature.

DETAILED DESCRIPTION OF THE INVENTION

Any particular method may be employed to produce the semiconductornanoparticles employed by the present COB-LED. However, it is preferredthat the semiconductor nanoparticles be produced by converting ananoparticle precursor composition to the material of the nanoparticlesin the presence of a molecular cluster compound under conditionspermitting seeding and growth of the nanoparticles on the clustercompound. The method may employ the methodology disclosed in U.S. Pat.Nos. 7,588,828, 7,803,423, 7,985,446, and 8,062,703; the entire contentsof these publications are hereby incorporated by reference in theirentireties.

The coordination about the final inorganic surface atoms in any core,core-shell or core-multi shell, doped or graded nanoparticle istypically incomplete, with highly reactive, non-fully coordinated atomsacting as “dangling bonds” on the surface of the particle, which canlead to particle agglomeration. This problem is typically overcome bypassivating (capping) the “bare” surface atoms with protecting organicgroups.

In many cases, the capping agent is the solvent in which thenanoparticles have been prepared and consists of a Lewis base compound,or a Lewis base compound diluted in an inert solvent such as ahydrocarbon. There is a lone pair of electrons on the Lewis base cappingagent that are capable of a donor type coordination to the surface ofthe nanoparticle; ligands of this kind include, but are not limited to,mono- or multi-dentate ligands such as phosphines (trioctylphosphine,triphenylphosphine, t-butylphosphine etc.), phosphine oxides(trioctylphosphine oxide, triphenylphosphine oxide etc.), alkylphosphonic acids, alkyl-amines (hexadecylamine, octylamine etc.),aryl-amines, pyridines, long chain fatty acids and thiophenes

In addition to the outermost layer of organic material or sheathmaterial (capping agent) helping to inhibit nanoparticle-nanoparticleaggregation, this layer may also protect the nanoparticles from theirsurrounding electronic and chemical environments, and provide a means ofchemical linkage to other inorganic, biological or organic material,whereby the functional group is pointing away from the nanoparticlesurface and is available to bond/react/interact with other availablemolecules. Examples include, but are not limited to, amines, alcohols,carboxylic acids, esters, acid chloride, anhydrides, ethers, alkylhalides, amides, alkenes, alkanes, alkynes, allenes, amino acids,azides, groups etc. The outermost layer (capping agent) of a quantum dotmay also consist of a coordinated ligand that processes a functionalgroup that is polymerizable and may be used to form a polymer layeraround the nanoparticle. The outermost layer may also consist of organicunits that are directly bonded to the outermost inorganic layer such asvia a disulfide bond between the inorganic surface (e.g., ZnS) and athiol capping molecule. These may also possess additional functionalgroup(s), not bonded to the surface of the particle, which may be usedto form a polymer around the particle, or for furtherreaction/interaction/chemical linkage.

It may be useful to incorporate quantum dots into a primary matrixmaterial forming beads. The primary matrix material is preferably anoptically transparent medium, i.e., a medium through which light maypass, and which may be, but does not have to be, substantially opticallyclear. The primary matrix material, may be a resin, polymer, monolith,glass, sol gel, epoxy, silicone, (meth)acrylate or the like, or mayinclude silica.

Any appropriate method may be used for incorporating the quantum dotsinto the primary matrix material for bead formation. One method involvesincorporating the quantum dots directly into the monomer solution duringbead formation. Afterwards, the monomer solution may be polymerizedusing any appropriate method resulting in the quantum dots beingrandomly dispersed within the polymer. In an alternative procedure, aligand exchange process may be carried out prior to the bead formingreaction. Quantum dots may be used as isolated from the reactionemployed to synthesize them and are thus generally coated with an inertouter organic ligand layer. Here one or more chemically reactive ligands(for example, this may be a ligand for the quantum dots which alsocontains a polymerizable moiety) are added in excess to a solution ofnascent quantum dots coated in an inert outer organic layer. Afterappropriate incubation time, the quantum dots are isolated, for exampleby precipitation and subsequent centrifugation, washed and thenincorporated into the mixture of reagents used in the bead formingreaction/process. Thereafter, during polymerization, the randomlydispersed quantum dots are covalently bonded into the polymer.

Another option for incorporating quantum dots into beads involvesdirectly depositing them into primary particles. For example, a solutionof quantum dots in a suitable solvent (e.g. organic solvent) may beincubated with a sample of primary particles. Subsequent removal of thesolvent leaves the quantum dots immobilized within the matrix materialof the primary particles. Additionally, one or more stability enhancingadditives may be added.

Once quantum dots are incorporated into a primary matrix, one may wantto provide an additional layer of an inorganic material on the quantumdot-containing primary particles, such as a metal oxide or metalnitride. One particularly effective method to add such a layer is AtomicLayer Deposition (ALD), although other suitable techniques may beemployed.

White light has been produced using quantum dots by utilizing a blue LEDin conjunction with red and green quantum dots. When some of theemission of the blue LED is absorbed and emitted by the green and redquantum dots, the resultant mix appears white. The present invention isdirected to overcoming, or at least reducing the effects of, one or moreof the problems set forth above.

FIG. 1 illustrates a printed circuit board (02) that forms the base fora QD-COB. On top of the printed circuit board, LED lights (04) (the“chips”) are attached using methods commonly understood in the art. Thisforms a traditional COB-LED. Once the LEDs are attached, they need to beencapsulated in some manner so that additional layers may be added tothe printed circuit board. In an embodiment, a ceramic ring (08) isattached to the printed circuit board (PCB) with the LEDs in the middle.A layer of LED encapsulant, such as silicone, is applied on top of theLEDs inside the encapsulating ring.

Once the LEDs are covered with silicone, they may be covered by anymatrix containing quantum dots. The matrix material may be a resin,polymer, monolith, glass, sol gel, epoxy, silicone, (meth)acrylate orthe like, or may include silica. The quantum dots suspended in thematrix material may be nascent quantum dots or bead quantum dots. Oncethe quantum dots are incorporated on the COB-LED, the dots may becapped, for example, by using a circular glass disc (010) and resin tobond the glass to the ceramic package. There are many ways in which thedots suspended on the package may be capped.

It is known that quantum dots degrade at high temperatures. In order toprotect the dots from extreme heat while attaching wires to the COB(06), a device has been produced that attaches to the pads of the COBwhile having the ability to connect the wires to the solder pads. Thewires are not soldered in the vicinity of the LED/quantum dot assembly.As such, the quantum dots are not exposed to the soldering heat duringsoldering. A view of the assembled device is illustrated in FIG. 2.

To prevent quantum dot degradation during operation, the drive currenton the COB is kept lower than would typically be used to produce maximumoutput.

FIGS. 3A and 3B are infrared thermal scans of an LED in a conventionalpackage. FIGS. 4A and 4B are infrared thermal scans of LED COB accordingto the invention. The COB was operated at 14.91v@98 mA; the conventionalLED package (in the same layout as the COB) was operated at 14.6v@94 mA(this configuration will henceforth be referred to as “LED”). The COBproduced 0.503W radiant flux; the LED produced 0.51W radiant flux.

As shown in FIG. 3A, the LED was at 26.6 degrees Celsius on initial turnon measured on the front face of film.

As shown in FIG. 4A, the COB exhibited 23.3 degrees Celsius on initialturn on measured on the front face of film.

After an hour operating time, the following results were observed: theLED face temperature had increased to 35.7 degrees Celsius, increasingby 9.1 degrees (FIG. 3B); and, the COB face temperature had increased to30.9 degrees Celsius, increasing by 7.6 degrees (FIG. 4B).

These results suggest that greater stability of quantum dots may beobtained by using the dots in a COB format. As illustrated graphicallyin FIG. 5, there is a performance drop of quantum dots with increasingtemperature (this data is representative only for previous studies withtemperature derating).

It is important to maintain a relatively low operating temperature ofquantum dots and the physical form of the COB allows for greater coolingtherefore allowing the quantum dots to be moved closer to the endpackage and maintain a higher radiant flux than if one were to use a PCBpopulated with LEDs and a remote lens.

The physical characteristics of the COB suit the needs of the quantumdots and may be used as an encapsulation aid to reduce exposure tooxygen.

The principle this data shows is that using quantum dots in conjunctionwith a COB format aids the performance of the material as measuredagainst traditional LED packages.

The foregoing description of preferred and other embodiments is notintended to limit or restrict the scope or applicability of theinventive concepts conceived of by the Applicants. It will beappreciated with the benefit of the present disclosure that featuresdescribed above in accordance with any embodiment or aspect of thedisclosed subject matter can be utilized, either alone or incombination, with any other described feature, in any other embodimentor aspect of the disclosed subject matter.

What is claimed is:
 1. A light-emitting device comprising: at least onelight emitting diode (LED) in a chip on board configuration mounted on aprinted circuit board (PCB); a cylindrical ring surrounding the LEDhaving a first end and an opposing second end and attached at the firstend to the PCB; an encapsulant within the cylindrical ring; a pluralityof quantum dot nanoparticles within the cylindrical ring in opticalcommunication with the LED; a substantially transparent cap attached tothe second end of the cylindrical ring.
 2. The light-emitting devicerecited in claim 1 wherein the LED is a blue LED.
 3. The light-emittingdevice recited in claim 1 wherein the plurality of quantum dotnanoparticles comprises red-emitting quantum dots and green-emittingquantum dots.
 4. The light-emitting device recited in claim 1 furthercomprising a seal between the substantially transparent cap and the sendend of the cylindrical ring.
 5. The light-emitting device recited inclaim 4 wherein the seal is a gas-tight seal.
 6. The light-emittingdevice recited in claim 1 wherein the substantially transparent capcomprises a glass disk.
 7. The light-emitting device recited in claim 1wherein the quantum dot nanoparticles comprise a capping agent.
 8. Thelight-emitting device recited in claim 7 wherein the capping agentcomprises a Lewis base compound.
 9. The light-emitting device recited inclaim 8 wherein the Lewis base compound is selected from the groupconsisting of phosphines, phosphine oxides, alkyl phosphonic acids,alkyl-amines, aryl-amines, pyridines, long chain fatty acids andthiophenes.
 10. The light-emitting device recited in claim 7 wherein thecapping agent consists of a coordinated ligand that possesses afunctional group that is polymerizable and has been used to form apolymer layer around the nanoparticle.
 11. The light-emitting devicerecited in claim 7 wherein the capping agent has an outermost layer thatconsists of organic units that are directly bonded to an outermostinorganic layer of the nanoparticle.
 12. The light-emitting devicerecited in claim 11 wherein the organic units that are directly bondedto an outermost inorganic layer of the nanoparticle are bonded via adisulfide bond between the inorganic surface and a thiol cappingmolecule.
 13. The light-emitting device recited in claim 1 wherein thequantum dots are incorporated into a primary matrix material formingbeads.
 14. The light-emitting device recited in claim 13 wherein theprimary matrix material is optically transparent.
 15. The light-emittingdevice recited in claim 14 wherein the primary matrix material isselected from the group consisting of a resin, a polymer, a monolith, aglass, a sol gel, an epoxy, silicone, and (meth)acrylate.
 16. Thelight-emitting device recited in claim 15 wherein the primary matrixmaterial further comprises silica.
 17. The light-emitting device recitedin claim 1 wherein the cylindrical ring is a ceramic ring.
 18. Thelight-emitting device recited in claim 1 wherein the encapsulantcomprises silicone.
 19. The light-emitting device recited in claim 1wherein the quantum dots are incorporated into a primary matrix andcomprise an additional layer of an inorganic material on quantumdot-containing primary particles.
 20. The light-emitting device recitedin claim 19 wherein the additional layer of inorganic material comprisesa metal oxide or a metal nitride.